Process for producing synthesis gas

ABSTRACT

A process for controlling the carbon conversion of a gasifier fuelled with a carbonaceous feedstock by mixing in biomass, the process comprising the steps of (a) pressurizing the biomass and carbonaceous feedstock; (b) introducing the biomass and carbonaceous feedstock into the gasification reactor vessel; (c) partially oxidizing the carbonaceous feedstock/biomass with a molecular oxygen-comprising gas to obtain a synthesis gas comprising carbon monoxide and hydrogen; (d) measuring the C02 content of the syngas and comparing with a pre-determined value range; (e) adjusting the biomass/carbonaceous feedstock ratio by changing the biomass feed rate; wherein said biomass and carbonaceous feedstock comprises from 10 wt % to 50 wt % of biomass and wherein the level of biomass is adjusted within this range to control the carbon conversion.

The present invention relates to a process for producing synthesis gasstream from a carbonaceous feedstock, which process includes biomassfuel as a means to control the carbon conversion at a given gasificationtemperature in a gasification process.

WO 03/012013 describes a process for the incomplete combustion ofdomestic waste to produce syngas. The process as described may alsoinclude controlling the temperature in various parts of the reactionspace by controlling the temperature of the oxygen-containing gas fedthereto, controlling the rate of feed of the waste and the resultingratio of the waste feed to the oxygen-containing gas feed, and thermallyinsulating the reation space.

JP2002194363 describes a method for the pressurized entrained bedgasification of coal, by which biomass can be used to efficiently lowerhigh heat generated by gasification of the coal, thereby reducing theoccurrence of problems with the accumulation and fusion of ash on theoperation of a pressurized entrained bed gasification furnace. Themethod is characterized by introducing coal particles into the lowerportion of the pressurized entrained bed gasification furnace to producethe high temperature coal gas, introducing a biomass fuel into the upperportion to bring into contact with the high temperature coal gas andgasifying the biomass fuel. Thereby, the height of an exhaust heatrecovery boiler of the gasification process can be controlled.

In the present invention a carbonaceous feedstock such as coal may bemixed with biomass prior to the mixture being introduced into the burnersection of the gasifier. The mixing of biomass with the coal in such amanner helps to ensure synergy between the oxygen rich biomass andoxygen lean carbonaceous feedstocks. Due to the high oxygen content inthe biomass feedstock, the requirement of a moderator gas to obtain fullconversion of the carbonaceous feedstock at a given gasificationtemperature is reduced and can even be eliminated at sufficient highfractions of biomass in the coal/biomass mixture. As moderator gas steamor carbon dioxide or a combination thereof is typically used.

An object of the present invention is to provide a simplified, energyefficient and renewable means of controlling the carbonaceous feedstockconversion by mixing in a biomass fuel. Biomass in general has a highoxygen content and can provide part of the oxygen required for fullconversion of the carbonaceous feedstock, which in turn reduces therequirement of a moderator gas, such as steam. A certain minimum biomassfraction in the biomass/carbonaceous feedstock mixture is required toeliminate the steam. This fraction is determined by the ultimatecomposition of the lignocellulosic biomass and carbonaceous feedstock. Atypical minimum fraction to get complete carbon conversion is 10-30% ofbiomass by weight in the carbonaceous feedstock/biomass mixture;however, any fraction between 10% to 50% may be used.

The above object is achieved with the following process.

A process for controlling the carbon conversion of a gasifier fuelledwith a carbonaceous feedstock by mixing in biomass, the processcomprising the steps of

(a) pressurizing the biomass and carbonaceous feedstock;(b) introducing the biomass and carbonaceous feedstock into thegasification reactor vessel;(c) partially oxidizing the carbonaceous feedstock/biomass with amolecular oxygen-comprising gas to obtain a synthesis gas comprisingcarbon monoxide and hydrogen;(d) measuring the CO2 content of the syngas and comparing with apre-determined value range;(e) adjusting the biomass/carbonaceous feedstock ratio by changing thebiomass feed rate;wherein said gasifier feed comprises from a minimum of 10 wt % to 50 wt%, preferably 10 wt % to 30 wt % of biomass and wherein the level ofbiomass may adjusted within this range as needed to control the carbonconversion and reduce or eliminate the moderator gas demand. Thesynthesis gas produced by this process may also contain water, hydrogensulphide and carbon dioxide.

Applicant found that by mixing 10 wt % to 50 wt %, preferably 10 wt % to30 wt % biomass with the carbonaceous feedstock, the temperature in thegasifier can be controlled using the oxygen/carbonaceous fuel ratio andthe carbon conversion can be controlled using the biomass/carbonaceousfuel ratio, thereby simplifying process control. The incorporation ofbiomass as described has the additional beneficial effects of saving onsteam and boiler feed water consumption and also reduces the overallcarbon footprint of the gasifier.

The invention will be described in more detail below.

In step (a) pressurization may be accomplished by a lock hopper systemor (solids) pumps. A lock hopper system is generally used to pressurizedry and solid feedstocks. A pump can be used to pressurize liquid orslurry (mixture of solids and liquid) feedstocks. A solids pump can alsobe applied to pressurize solid feedstocks.

In step (b) pulverized or liquid/slurry biomass can be mixed with thecarbonaceous feed or fed separately to the gasifier. A separate feedlinefor the biomass to the gasification reactor is preferred as this willreduce the response time on a step change from the carbon conversioncontroller significantly. Mixing of the biomass with the carbonaceousfeed can be done at several locations in the fuel preparation andpressurization section.

In step (c) any type of coal or oil burner configurations can be used.When the biomass is supplied separately to the gasification reactor thenthe biomass may also be introduced through a nozzle in the burner areaof the gasification reactor. The oxygen for the partial oxidation of thebiomass will then need to be supplied via the carbonaceous fuel burner.

In step (c) the carbonaceous/biomass feedstock is subjected to partialoxidation with a molecular oxygen comprising gas. The partial oxidationis preferably performed at a temperature of between 1000 and 1800° C.and more preferably at a temperature between 1200 and 1800° C. Thepressure at which partial oxidation is performed is preferably between0.3 and 12 MPa and preferably between 3 and 10 MPa. When an ashcontaining feedstock is used the temperature conditions are so chosenthat a slag layer will form on the interior of the reactor vessel inwhich the partial oxidation takes place.

In step (d) the CO2 content is preferably measured in cleaned and coldsyngas, for example downstream of the wet scrubber. A fast measurementis preferred to minimize the time between a step change in CO2concentration and the control action. An infrared based analyser is anexample of such a fast measurement device.

In step (d) the CO2 content of the syngas is compared with a pre-definedvalue. In step (e) a controller will subsequently adjust thebiomass/coal ratio by preferably changing the set-point of the biomassfeed rate controller.

The carbonaceous feedstock is preferably coal, as for exampleanthracite, brown coal, bitumous coal, and sub-bitumous coal. Examplesof alternative carbonaceous feedstocks are petroleum coke, peat andheavy residues as extracted from tar sands or the asphalt fraction asseparated from said residues in a de-asphalting process. Residues fromrefineries such as residual oil fractions boiling above 360° C.,directly derived from crude oil, or from oil conversion processes suchas thermal cracking, catalytic cracking, hydrocracking etc may also beused as the carbonaceous feedstock.

Any biomass derived feedstocks containing low moisture content oftypically lower than 20 wt % and which can be ground to particles withsizes between 10 and 1000 micron are suitable solid biomass feedstockscomprising the 10 wt % to 50 wt % biomass component of the carbonaceousfeedstock. Feedstocks obtained by torrefaction of a biomass source aresuitable as well and are preferred at higher biomass fractions in themixture. Torrefaction is preferably combined with a compression orpelletization step in order to make the biomass feed more suited for agasification process wherein the biomass feed is supplied in a so-calleddry form or slurry form when torrefied particles are mixed with acarbonaceous liquid. Torrefaction of biomass source material is wellknown and for example described in M. Pach, R. Zanzi and E. Bjornbom,Torrefied Biomass a Substitute for Wood and Charcoal. 6th Asia-PacificInternational Symposium on Combustion and Energy Utilization. May 2002,Kuala Lumpur and in Bergman, P.C.A., “Torrefaction in combination withpelletisation—the TOP process”, ECN Report, ECN-C-05-073, Petten, 2005.

Another suitable solid biomass fuel in the present process is obtainedby drying and slow pyrolysis of a biomass source. In slow pyrolysisprocesses a solid char feed component is typically obtained. Slowpyrolysis is well known and for example described In: “Pyrolysis andOther Thermal Processing”. US DOE 14-08-2007.

A suitable liquid or solid biomass feedstock for use in the presentprocess is obtained by drying and flash pyrolysis of a biomass source.In flash pyrolysis processes a solid char and a liquid biomass feedcomponent are typically obtained. Both can be used as feedstock for thegasification process. Flash pyrolysis is well known and for exampledescribed in EP-A-904335; in Dinesh Mohan, Charles U. Pittman, Jr., andPhilip H. Steele. Pyrolysis of Wood/Biomass for Bio-oil: A CriticalReview. Energy & Fuels 2006, 20, 848-889; and in E. Henrich: Cleansyngas from biomass by pressurised entrained flow gasification ofslurries from fast pyrolysis. In: Synbios, the syngas route toautomotive biofuels, conference held from 18-20 May 2005, Stockholm,Sweden (2005). The present invention is also directed to embodimentswherein a so-called biomass slurry is used as feedstock. The slurry canbe obtained by mixing the pyrolysis oil and char.

Suitable biomass sources are weeds or residues of the agriculturalindustry. Examples of suitable residue products are streams generated inthe palm oil industry, corn industry, bio-diesel industry, forestryindustry, wood processing industry and paper industry. Certain biomassis relatively expensive—for example, wood pellets. So, in order to keepoperational costs as low as possible, less costly biomass sources arepreferred, such as oil palm trunks and saw dust. However, the lowquantities of biomass needed for the replacement of the moderator steamincreases the chance for the application of biomass from waste streams,which are normally cheaply available in small quantities.

The present invention now provides for a process whereby the partialoxidation of a carbonaceous feedstock is performed in an efficientmanner thereby obtaining synthesis gas with a reduced carbon footprintsuited for catalytic conversion reactions. An especially interestingcatalytic conversion reaction is a hydrocarbon synthesis process. In ahydrocarbon synthesis process, synthesis gas is catalytically convertedinto hydrocarbon compounds ranging from methane to high molecular weightmolecules comprising up to 200 carbon atoms, or, under particularcircumstances, even more. An example of a hydrocarbon synthesis processis the Fischer-Tropsch process, described in e.g. WO 02/02489, WO01/76736, WO 02/07882, EP 510771 and EP 450861.

Step (c) may be performed by means of various gasification processes,such as for example the so-called moving bed process, fluid bed gasifierprocess or the entrained-flow gasifier process as for example describedin Gasification, by Christofer Higman and Maarten van der Burgt, 2003,Elsevier Science, Burlington Mass., Pages 85-128. Preferably anentrained-flow gasifier is used because the process can handle a largevariety of feedstock and because a tar-free synthesis gas is prepared.In such a process the feedstock and oxygen are introduced into thereactor co-currently, preferably by means of a suitable burner. Examplesof suitable burners and their preferred uses are described in U.S. Pat.No. 4,510,874 and in U.S. Pat. No. 4,523,529. The operating conditionsare such that the process is operated in a slagging mode, which meansthat the operating temperature is above the ash melting point. Suitablythe carbonaceous feedstock and the molecular oxygen comprising gas isconverted to synthesis gas by providing said reactants to a burner aspresent in a gasification reactor at a pressure of between 3 and 10 MPaand preferably at a pressure between 4 and 8 MPa. The operatingtemperature is suitably between 1200 and 1800° C. The synthesis gas ispreferably cooled to a temperature of below 1000° C., preferably below500° C. with either direct quenching with evaporating water, directquenching with a methanol-water mixture, by indirect heat exchangeagainst evaporating water or combination of such cooling steps. Slag andother molten solids are suitably discharged from the gasificationreactor at the lower end of the said reactor.

A solid carbonaceous feedstock/biomass feed mixture may be provided tothe burner of the entrained flow gasifier reactor as a slurry in water.Coal slurry feeding processes are for example described in EP-A-168128.Preferably the solid carbonaceous feedstock/biomass feedstock isprovided to the burner in a gas-solids mixture comprising the solid feedin the form of a powder and a suitable carrier gas. Suitable carriergasses are nitrogen, carbon dioxide, natural gas or synthesis gas, i.e.a mixture comprising of CO and H₂. The carrier gas is preferably carbondioxide. The use of this carrier gas is for example described inWO-A-2007042562.

FIG. 1 shows a preferred embodiment of the present invention.

FIG. 1 schematically shows a system for producing and cleaning synthesisgas. In a gasification reactor 9 a carbonaceous feed, a biomass feed andan oxygen containing feed are introduced. The oxygen and carbonaceousstream are fed via lines 19 and respectively. The biomass can be fedseparately via line 25 to the gasification reactor 9 but can also bemixed with the carbonaceous stream in the feeding and pressurizationsection 7 via line 5. The carbonaceous stream 8 and biomass stream 25are at least partially oxidised in the gasification reactor 9, therebyobtaining a raw synthesis gas 10 and a slag. To this end usually severalburners (not shown) are present in the gasification reactor 9. Thebiomass stream 25 can also be introduced in the gasification reactor 9via nozzles in the burner area of gasification reactor, for example atburner level. Then the oxygen for the partial oxidation of the biomasswill be added to the burner oxygen of the carbonaceous stream 8.

The carbon conversion of the gasifier is controlled with controller 18.Input for the carbon conversion controller is the measured CO2 contentof the syngas 16 downstream of the wet scrubber 15 with the CO2 analyser17. As soon as the CO2 content drops below a pre-determined value thanthe biomass/coal ratio needs to be increased by changing the biomassfeed 3 and increasing the flow rate of line 5 or 25. As soon as the CO2concentration exceeds a maximum pre-determined value, then thebiomass/coal ratio can be reduced by changing the biomass feed 3 anddecreasing the flow rate of line 4 or 25. The CO2 analyser and benefitsfor controlling the steam and oxygen to carbon (O/C) ratio are describedin WO2008125556A1 and WO200768684A2.

An advantage for the use of biomass to control the carbon conversion isthat a CO2 neutral feedstock is used, which reduces the CO2 footprint ofthe plant. In addition this application reduces the high pressure steamand high quality boiler feed water requirements for the gasificationisland.

The produced raw synthesis gas is fed via line 10 to a cooling section11; herein the raw synthesis gas is usually cooled to about 200-400° C.The cooling section 11 may be an indirect heat exchanger or a quenchvessel. In case of a quench vessel liquid water is preferably injectedvia line 23 into a synthesis gas stream. Liquid water is preferablyinjected in the form of a mist.

The cooling section 11 can be integrated in the gasification reactorpressure vessel at the bottom of the reactor or installed as a separatevessel. When the syngas and slag both exit at the bottom of thegasification reactor then the cooling section 11 is integrated in thepressure vessel and both the syngas and fly ash and slag will enter thecooling section. When the syngas leaves the gasification reactor 9 atthe top and slag at the bottom then only syngas with fly ash will enterthe cooling section that is installed as separate section downstream ofthe reactor.

As shown in the embodiment of FIG. 1, the raw synthesis gas leaving thecooling section 10 is further processed. To this end, it is fed via line12 into a dry or wet solids removal unit 13 to at least partially removeash in the raw synthesis gas. As the solids removal unit 13 is known tothose of ordinary skill in the art, it is not further discussed here.The ash is removed from the solids removal unit via line 24. After thesolids removal unit 13 the raw synthesis gas may be fed via line 14 to awet gas scrubber 15 and subsequently via line 16 to battery limits.After further syngas treating, the residual syngas can be used forseveral applications like power generation, production of H2,fertilizer, Fischer Tropsch liquids and other chemicals.

The reduction of the moderator steam with the addition of biomass tocoal was modelled using a computer simulation. For the gasification of aselected Drayton coal, approximately 76 kg steam per tonne coal will berequired to obtain full carbon conversion at a gasification temperatureof 1500° C. The addition of coal with wood will gradually reduce themoderator requirement as shown in table 1. With the replacement of >25%of the coal with wood, moderator steam can be eliminated.

TABLE 1 Typical moderator consumption at different biomass fractions inbiomass/coal mixture Coal in mixture 100% 90% 80% 70% Biomass in mixture0% 10% 20% 30% Gasification conditions Temperature ° C. 1500 1500 15001500 Pressure bara 41 41 41 41 GASIFIER INPUT Carrier kg/s 2.7 2.7 2.72.7 Moderator steam kg/s 2.1 1.1 0.2 0.0 O2 kg/s 22.2 21.3 20.4 19.7Fuel kg/s 27.6 27.6 27.6 27.6 GASIFIER OUTPUT Syngas kg/s 49.9 48.5 47.146.6 Slag kg/s 6.0 5.5 4.9 4.4

1. A process for controlling the carbon conversion of a gasifier fuelledwith a carbonaceous feedstock by mixing in biomass, the processcomprising the steps of (a) pressurizing the biomass and carbonaceousfeedstock; (b) introducing the biomass and carbonaceous feedstock intothe gasification reactor vessel; (c) partially oxidizing thecarbonaceous feedstock/biomass with a molecular oxygen-comprising gas toobtain a synthesis gas comprising carbon monoxide and hydrogen; (d)measuring the CO2 content of the syngas and comparing with apre-determined value range; (e) adjusting the biomass/carbonaceousfeedstock ratio by changing the biomass feed rate; wherein said biomassand carbonaceous feedstock comprises from 10 wt % to 50 wt % of biomassand wherein the level of biomass is adjusted within this range tocontrol the carbon conversion.
 2. A process according to claim 1,wherein the carbonaceous feedstock and biomass are separately introducedinto the gasification reactor.
 3. A process according to claim 1,wherein the carbonaceous feedstock and biomass are introduced in thegasification reactor as a mixture.
 4. A process according to claim 1,wherein the feedstock to the gasifier comprises from 10 wt % to 30 wt %of biomass fuel.
 5. A process according to claim 1, wherein the biomassis a solid biomass as obtained by torrefaction.
 6. A process accordingto claim 1, wherein the biomass is a solid biomass as obtained by slowpyrolysis of a biomass source.
 7. A process according to claim 1,wherein the biomass is a solid biomass as obtained by flash pyrolysis ofa biomass source.
 8. A process according to claim 4, wherein the biomassis a liquid biomass as obtained by flash pyrolysis of a biomass source.9. A process according to claim 4, wherein the biomass is a slurrybiomass consisting of a mixture of pyrolysis oil and char as obtained byflash pyrolysis of a biomass source.
 10. A process according to claim 1,wherein the carbonaceous feedstock is coal.
 11. A process according toclaim 1, wherein the carbonaceous feedstock is oil residue.
 12. Aprocess according to claim 1, wherein in step (c) the carbonaceousfeedstock/biomass fuel mixture and the molecular oxygen comprising gasare converted to synthesis gas by providing said reactants to a burneras present in a gasification reactor at a pressure of between 1 and 10MPa.
 13. A process according to claim 1, wherein in step (d) the CO2content is measured using an infrared based analyser.
 14. A processaccording to claim 1, wherein, in step (e) a controller will adjust thebiomass/coal ratio by changing the set-point of a biomass feed ratecontroller.